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The stellar initial mass function (IMF), which is often assumed to beuniversal across unresolved stellar populations, has recently been suggested tobe "bottom-heavy" for massive ellipticals. In these galaxies, the prevalence ofgravity-sensitive absorption lines (e.g. Na I and Ca II) in their near-IRspectra implies an excess of low-mass ($m <= 0.5$ $M_\odot$) stars over thatexpected from a canonical IMF observed in low-mass ellipticals. A directextrapolation of such a bottom-heavy IMF to high stellar masses ($m >= 8$$M_\odot$) would lead to a corresponding deficit of neutron stars and blackholes, and therefore of low-mass X-ray binaries (LMXBs), per unit near-IRluminosity in these galaxies. Peacock et al. (2014) searched for evidence ofthis trend and found that the observed number of LMXBs per unit $K$-bandluminosity ($N/L_K$) was nearly constant. We extend this work using new andarchival Chandra X-ray Observatory (Chandra) and Hubble Space Telescope (HST)observations of seven low-mass ellipticals where $N/L_K$ is expected to be thelargest and compare these data with a variety of IMF models to test which areconsistent with the observed $N/L_K$. We reproduce the result of Peacock et al.(2014), strengthening the constraint that the slope of the IMF at $m >= 8$$M_\odot$ must be consistent with a Kroupa-like IMF. We construct an IMF modelthat is a linear combination of a Milky Way-like IMF and a broken power-lawIMF, with a steep slope ($\alpha_1=$ $3.84$) for stars < 0.5 $M_\odot$ (assuggested by near-IR indices), and that flattens out ($\alpha_2=$ $2.14$) forstars > 0.5 $M_\odot$, and discuss its wider ramifications and limitations.

Testing the Universality of the Stellar IMF with Chandra and HST